Analogue circuits in an RF receiver exhibit comparatively high power consumption in order to provide a good linearity regarding signals of high input power as well as good sensitivity regarding signals of low input power.
Linearity of an amplifier refers to the consistent transfer function of an amplifier for small or large signals. Measures of degradation of linearity are for instance signal compression or inter-modulation/distortion of large input signals. Inter-modulation/distortion refers to the amplitude modulation of signals containing two or more different frequencies in a system. The inter-modulation between each frequency component will form additional signals at frequencies that are not just at harmonic frequencies of either but also at the sum and difference frequencies of the original frequencies and multiples of those sum and difference frequencies. Inter-modulation is rarely desirable in radio processing. For radio reception, strong adjacent channels can interfere with the desired signal resulting in loss of reception.
Sensitivity of an RF receiver circuit may be strongly correlated to the signal-to-noise ratio (SNR) caused by components in a radio frequency signal chain. Measures of degradation of the SNR are for instance noise figure (NF) or noise factor (F).
In order to provide good linearity to large input power signal levels and good sensitivity to low input power signals, analogue circuits in radio receivers exhibit a comparatively high current consumption. For mobile application scenarios this may lead to unacceptably short battery life. Reducing the current consumption of the RF receiver circuit either degrades sensitivity, linearity, or both.
Moreover, if the gain of a receiver is not properly adjusted for incoming signal strength of a desired signal, the receiver may be improperly insensitive or overly sensitive. An insensitive receiver is likely to be a poor receiver of a low level desired signal. An overly sensitive receiver is more susceptible to nearby interferers.
There already exist automatic gain control (AGC) circuits that are operable to decrease the gain of the receiver on demand. Hence, in the presence of a comparatively large input signal comparatively good linearity specifications can be achieved, while increasing gain when a comparatively small input signal or no further signals are present.
A conventional automatic gain control is for instance described in U.S. Pat. No. 7,664,211 B2.
Generally, an RF receiver featuring AGC typically comprises a low-noise amplifier (LNA), a down-converter mixer, a low-pass or band-pass filter, an analogue-to-digital converter (ADC) and a demodulator. The AGC circuit itself typically comprises a peak detector by means of which sensing of a received RF signal can be conducted. The received or detected RF signal is then to be compared with a threshold value provided by a threshold value generator, typically implemented in form of a digital-to-analogue converter (DAC).
Comparison of the signals of the peak detector and the threshold value generator is typically conducted by means of a comparator of an AGC circuit. The output of the comparator is then coupled with an up/down counter whose output is fed back to the LNA in order to adjust the gain of the LNA in discrete steps.
If for instance the RF input power is comparatively large, the comparator causes the up/down counter to count up, thereby decreasing the gain of the LNA. In this way, the linearity of the receiver can be increased allowing reception of a desired signal in the presence of blocking signals. In another scenario, wherein the RF input power is small, the up/down counter counts down, thus increasing the gain of the LNA. This increases sensitivity of the receiver allowing to receive comparatively small or weak signals, e.g. broadcasted by a source which is located far away.
Typically, separate thresholds are implemented to create a window for counting up and for counting down. Such a window can be in principle implemented by means of two separate threshold value generators with two comparators. Alternatively, two different thresholds could also be implemented by operating the comparator and the threshold value generator in difficult threshold modes in a temporal alternating way, hence by driving the threshold value generator and the respective comparator in separate comparison cycles.
Even though existing AGC designs allow for a substantive reduction of power consumption a problem may be encountered when the LNA is in a low-gain state and when an RF signal of comparatively high power is abruptly no longer available. Then, a comparatively small threshold voltage needs to be used on the threshold value generator to trigger the up/down counter to count back down. Here, generic offsets in the peak detector, the threshold value generator, hence in the DAC and in the comparator itself may become significant compared to the small threshold voltage. In a critical situation, the counter will not be triggered to count down in order to raise sensitivity of the RF receiver.